This invention relates to a speed control system of traction motors for electric vehicles and more particularly, to a chopper control circuit for use in a speed control system of traction motors wherein a controllable chopper is employed to intermittently switch a D.C. voltage to be supplied to the traction motor.
The chopper for chopping the D.C. voltage fed to the traction motor usually comprises a thyristor which is controllable by a pulse for initiating its conduction and a pulse for stopping its conduction. Usually, these pulses are formed from a pulse produced by a monostable multivibrator. This monostable multivibrator is controlled by a periodical pulse produced by a pulse generator circuit.
It is necessary for the monostable multivibrator used for chopper control to control the chopper such that the duty factor of the D.C. voltage fed to the traction motor is controlled by the chopper with extremely high accuracy. Typically, a monostable multivibrator for this purpose is constituted with a pair of transistors. Such a monostable multivibrator suffers from failure of accurate control for the duty factor since transistors are sensitive to temperature and ambient disturbance and for this reason, they are unstable. In addition, with the monostable multivibrator utilizing paired transistors, it is impossible to reduce the reset time below a certain limit with the result that the control for the duty factor is confined within a narrow range.